Commercially available laser discs (LD) and 5.25-inch magneto-optic discs are constructed by providing information carrying means on a surface of a resin substrate of 1.2 mm thick and joining another resin substrate to the resin substrate with an adhesive layer so that the information carrying means is disposed inside. The adhesive layer is typically of a hot melt adhesive as disclosed in JP-A 6536/1983. Compact discs (CD) are constructed to a single substrate structure by providing information carrying means on a resin substrate of 1.2 mm thick and providing a reflective layer and a resinous protective layer thereon.
To meet the necessity to focus a laser beam at a smaller spot in order to increase the information carrying density of an optical disc, it is now attempted to use a resin substrate with a thickness of 0.6 mm. When the thickness of a resin substrate is reduced to one unit of the conventional substrate, the mechanical strength is reduced to a quarter of the conventional substrate. This prohibits the use of a single substrate as in CDs. It is thus proposed to use the construction that a pair of resin substrates are joined together as in LDs.
Optical discs comprising a pair of resin substrates of 0.6 mm thick joined together are expected to find use as digital video discs (DVD) since they have a very high information carrying density and a high capacity despite the small diameter equal to that of CDs. Because of the small diameter, the use of such optical discs in automobiles is being considered.
In order that optical discs be operated in automobiles, countermeasures must be taken against high temperature, vibration and other automotive environmental factors. Since the automotive interior can be heated to 80.degree. C. or higher when exposed to sunlight, the adhesive for joining substrates is required to be fully heat resistant. Prior art hot melt adhesives commonly used in the optical disc field tend to soften to lower a bonding power in a hot environment, leaving the problem that optical discs lose mechanical strength.
One heat resistant adhesive is an adhesive of reaction curing type as disclosed in JP-A 110350/1986, for example. The adhesive of reaction curing type is well heat resistant in that once cured, it does not soften as do hot melt adhesives even when the surrounding temperature is elevated. Adhesives of UV curing type are often used among adhesives of the reaction curing type because they cure within a short time.
The optical discs using heat resistant adhesives, however, were found to be less stable against vibration. Since heat resistant adhesives have a high Young's modulus at about room temperature, the adhesive layer cannot accommodate the vibration transmitted to the optical disc during rotation. Vibration causes dynamic axial runout to the optical disc, giving rise to a problem upon writing/reading operation, especially upon writing operation. In particular, discs using a substrate having a greater ratio of diameter to thickness are largely affected by vibration.
It is noted that those hot melt adhesives having a high melting point are very heat resistant. They were also found to be less stable against vibration.